Power plant with combustion of a fuel in a fluidized bed

ABSTRACT

The invention relates to a power plant with combustion of a fuel at a pressure exceeding the atmospheric pressure in a fluidized bed (18) of particulate material, a so-called PFBC power plant. A combustor (12) is enclosed within a pressure vessel (10) and surrounded by compressed combustion air. Ash chambers (44) in the lower part of the combustor (12) are enclosed in one or more spaces (50) with walls (52, 53) which are suitably designed as plane, water-cooled panel walls. The ash chambers (44) are designed with uncooled walls (62). Pressure equalizing openings (64) are provided in the ash chamber walls (62), which openings equalize any pressure differences which may arise between the ash chambers (44) and the surrounding space (50). The walls (52, 53) which form the space surrounding the ash chambers absorb the pressure difference between the ash chambers (44) and the space (32) between the combustor (12) and the pressure vessel (10).

TECHNICAL FIELD

The invention relates to a power plant with combustion of a fuel at apressure exceeding the atmospheric pressure in a fluidized bed ofparticulate material in a combustor placed inside a pressure vessel andsurrounded by compressed combustion air in the space between thepressure vessel and the bed vessel, a so-called PFBC power plant. PFBCare the initial letters of the English expression Pressurized FluidizedBed Combustion.

BACKGROUND ART AND THE TECHNICAL PROBLEM

Combustors with combustion of a fuel, usually coal, in a fluidized bedare advantageously constructed with a so-called open bottom consistingof parallel air distributing tubes for combustion air to a combustionspace above the bottom and with one or--in large combustors--severalfunnel-shaped ash chambers below the air distributing bottom tubes.Residual products, residues from the fuel and consumed bed materialabsorbent, pass in gaps between the air distributing tubes to the ashchamber or ash chambers. The gaps between the tubes should be of such asize that slag lumps formed during normal operation may pass freelythrough the gaps so as not to disturb the combustion. It is desirablethat the downwardly-directed flow to the ash chamber is evenlydistributed over the cross section of the combustor. To obtain a smalloverall height and a uniform material flow, therefore, large combustorsare formed with a plurality of ash chambers with a rectangular crosssection nearest the combustor bottom. The ash chambers may have theshape of a pyramid turned upside down, or of a funnel of rectangularcross section which changes into a circular cross section so that itslowermost part has the shape of a cone. A power plant with a combustorof this kind is described in greater detail in European patentapplication A1 289 974.

Between the ash chambers and the surrounding space in the pressurevessel, the pressure difference may amount to about 1 bar, which meansthat they are subjected to great forces. Since the temperature is highalso in the ash chamber, especially in the uppermost part, the ashchamber walls have been constructed as cooled panel walls which aretraversed by cooling water to attain a satisfactory strength.Constructing the ash chambers with water-cooled panel walls in acombustor with a plurality of ash chambers complicates the design andentails high costs of manufacture and erection.

SUMMARY OF THE INVENTION

The present invention aims to simplify and render less expensive the ashchamber section of the combustor. According to the invention, thecombustor is designed with a plurality of ash chambers which are all orin groups enclosed in spaces which are separated from the space betweenthe combustor and the pressure vessel. In this way the ash chamber wallswill not be subjected to forces caused by a pressure difference betweenthe ash chamber and the surroundings. By providing pressure equalizingopenings in the ash chamber walls, pressure equalization is achievedbetween the ash chamber and the surrounding space when the operatingpressure of the plant changes upon variations of the load.

The ash chamber walls only support the load from the bed material andresidual products from the combustor and sufficient strength may beimparted to them also at a relatively high wall temperature. Therefore,they do not have to be designed as cooled walls, which entails a simpledesign and a low cost. The pressure difference between the ash chamberand the space between the combustor and the pressure vessel is absorbedby the walls around the space which surrounds the ash chamber. These areplane and simple to manufacture even if designed, as water-cooled, panelwalls. The forces acting on the walls are absorbed partly as bendingstresses in the walls and partly by supports connecting the walls toeach other and/or by supports connecting the walls to a framework.

Other characteristics of the invention will be clear from the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawing, wherein

FIGS. 1 and 2 schematically show two embodiments of a PFBC power plantto which the invention is applied,

FIG. 3 shows a section according to A--A in FIG. 2,

FIG. 4 shows a section according to B--B in FIG. 3,

FIG. 5 shows a section according to C--C in FIG. 2, and

FIG. 6 shows a perspective sketch, partly in section, of the lower partof a combustor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the figures, 10 designates a pressure vessel. In it are placed acombustor 12 and a cleaning plant 14, symbolized by a cyclone, forseparation of dust from combustion gases generated upon combustion of afuel in a fluidized bed 16 in the combustion space in the combustor 12.The combustion gases are collected in the freeboard 20, are cleaned inthe cleaning plant 14 and are passed in the conduit 22 to the turbine24. The turbine 24 drives a generator 26 and a compressor 28 which, byway of the conduit 30, supplies the space 32 between the pressure vessel10 and the combustor 12 and the cleaning plant, 14 with compressedcombustion air. The combustion space 18 of the bed vessel accommodatestubes 34 for generation of steam to a steam turbine (not shown). Fuel issupplied to the combustor 12 through the conduit 36 and nozzles (notshown).

The combustor 12 is provided with an open bottom 38 consisting of anumber of elongated air distributing tubes 40 with air nozzles 42 forthe supply of combustion air for fluidization of the bed 16 andcombustion of the supplied fuel. This bottom 38 divides the combustor 12into an upper part with the combustion space 18 and the freeboard 20,and a lower part consisting of a number of funnel-shaped ash chambers44. In large combustors the provision of a plurality of ash chambersmeans that no complicated internal devices are needed in the ashchambers for controlling the ash flow towards an outlet. The necessaryheight for a good ash flow is reduced. The stresses in the ash chamberwalls are low because of a small volume of material in each one of theash chambers. Between the tubes 40 there are openings 46 in which bedmaterial and residual products may pass to the ash chambers 44 and bedischarged through conduits 48 and discharge devices (not shown). Theash chambers 44 are funnel-shaped with an upper rectangular part whichis connected to one single conical part.

In the embodiment shown in FIG. 1 all ash chambers 44 are enclosedwithin a common space 50 which is surrounded by water-cooled panel walls52 and a water-cooled panel bottom 53. Air from the space 32 is suppliedto the tubes 40 through the transversely extending channel 54 withcooled walls 56. The ash chambers 44 are supplied with special, cooledair for cooing the ashes via tubes 58 with nozzles 60. The walls 62 ofthe ash chambers 44 are provided with pressure equalizing openings 64which equalize the pressure between the ash chamber 44 and thesurrounding space 50. These openings prevent a significant pressuredifference from arising between the ash chamber 44 and the space 50.Since the ash chamber, walls 62 need not absorb, forces by the pressuredifference but only from the material in them, which is cooled by air toa certain extent, it is possible to design the ash chamber wallsuncooled. This is of great value since they have a complicated shape andan embodiment with water-cooled panel walls entails a much moreexpensive design. The walls 52 around the space 50, which absorb thepressure difference instead of the ash chamber walls 62, are plane,simple to manufacture and may easily be supported or provided withframes for absorbing forces arising as a result of the pressuredifference. They may be designed uncooled or as water-cooled panel wallsas in FIG. 1. Openings 64 in the ash chamber wall 62 are designed as ashlocks.

In the embodiment shown in FIGS. 2-5 the ash chambers 44 are dividedinto two parallel groups. These groups are each enclosed in space 50.The confronting walls 52a form a narrow duct 66 which at its ends isdefined by end plates 68 and by a bottom 70 with openings 72. Air fromthe space 32 is supplied to the tubes 40 through the duct 66. Start-upburners or start-up combustors 74 may be provided in the openings 72.The duct 66 and the air tubes 40 communicate with each other by way ofsleeves 76 (thermosleeves) which allow thermal movement between thetubes 40 and the duct 66.

The forces on the plane walls 52 which arise because of the pressuredifference, up to about 1 bar, between the space 50 and the space 32 aregreat. To reduce the bending stresses in the panel walls, these areconnected to each other by means of load-supporting supports 78 and/orconnected by means of load-absorbing frameworks (not shown).

I claim:
 1. A power plant with combustion of a fuel, primarily coal, ata pressure exceeding the atmospheric pressure in a fluidized bed ofparticulate material, comprising:a combustor enclosed in a pressurevessel and surrounded by compressed combustion air in a space betweenthe combustor and the pressure vessel, a plurality of parallel airdistributing tubes with nozzles which form as bed bottom and divide thecombustor into an upper part which includes a combustion space and afreeboard above the fluidized bed, and a lower part which forms at leasttwo ash chambers for the discharge of ashes and consumed bed material,gaps between the air distributing tubes, through which ashes and bedmaterial may pass from the combustion space to the ash chambers, a ductlocated below the bed bottom and between the ash chambers from which theair distributing tubes are supplied with combustion air from the spacebetween the combustor and the pressure vessel for fluidization of thebed and combustion of the fuel in the bed, and wherein at least one ofthe ash chambers is enclosed within a space which is delimited from thespace for compressed combustion air in the pressure vessel.
 2. A powerplant according to claim 1 comprising a plurality of spaces which areseparated from the space for compressed air in the pressure vessel andwherein each of these spaces surrounds at least one of the ash chambers.3. A power plant according to claim 2, wherein the walls of the ashchambers are provided with openings for equalizing the pressure betweenthe ash chambers and the surrounding space.
 4. A power plant accordingto claim 1, wherein:the combustor is rectangular, and has a plurality ofseparate ash chambers arranged in at least two parallel rows, theparallel rows of ash chambers are each enclosed within an elongatedspace with cooled walls, and wherein the air distributing tubescommunicate with a duct formed between said elongated spaces.
 5. A powerplant according to claim 4, wherein:said duct between the elongatedspaces with cooled walls is defined from the space formed between thepressure vessel and the combustor by a bottom wall and end walls,wherein openings are provided at the bottom, and wherein start-upburners or start-up combustors are provided in or adjacent to saidopenings.
 6. A power plant according to claim 5, wherein force-absorbingelements are provided which connect together the cooled walls of thespaces surrounding the ash chambers.
 7. A power plant according to claim1 wherein the walls of the ash chambers are provided with openings forequalizing the pressure between the ash chambers and the surroundingspace.